1. Field of the Invention
The present invention relates generally to the field of semiconductor manufacturing and, in particular, to forming metal carbide thin films using atomic layer deposition (ALD).
2. Description of the Related Art
The integration level of components in integrated circuits is increasing, producing a need for smaller components, including interconnects. Design rules are dictating a feature size less than or equal to 0.2. μm. This makes film coverage in deep vias important but difficult to obtain.
The trend of decreasing feature size is evident, for example, in memory circuits or devices such as dynamic random access memories (DRAMs), flash memory, static random access memories (SRAMs), ferroelectric (FE) memories, and integrated circuit components, such as gate electrodes and diffusion barriers in complementary metal oxide semiconductor (CMOS) devices.
Metal carbides have found widespread use in the electronics industry, from gate electrodes to diffusion barriers. For example, tantalum carbide (TaC) is a low resistivity metal that is commonly used as an n-type metal oxide semiconductor (NMOS) gate electrode. Further, TaC has been found to be effective at inhibiting electromigration of noble metal atoms at the interface between metal interconnects and metal lines. As another example, metal carbide films have been used as barrier layers in damascene and dual damascene structures.
Transition metal carbides typically include one or more metals of groups 4, 5, 6, 7, 8, 9, 10, or 11 of the periodic table. Transition metal carbides are generally relatively inert, have very high melting points, are extremely hard and wear resistant, and have high thermal conductivity and metal-like electrical conductivity. For these reasons, transition metal carbides have been proposed for use as low resistance diffusion barriers in semiconductor fabrication (see, e.g., international patent application WO 00/01006; U.S. Pat. No. 5,916,365).
Transition metal carbides can have a wide range of compositions. Ordered and disordered carbon deficient forms exist, of which the tungsten carbides, WCx, are examples. In these forms, carbon resides in the interstitial cavities between metal atoms. General information about metal carbides can be found, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th Edition, Vol. A5, VCH Verlagsgesellschaft, 1986, pg. 61-77, and in the Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, Vol. 4, John Wiley & Sons, Inc., 1992, pg. 841-878.
Deposition methods available for forming metal carbide films or thin films include chemical vapor deposition (CVD), physical vapor deposition (PVD) and atomic layer deposition (ALD), which is sometimes called atomic layer epitaxy (ALE).
A CVD method of depositing tungsten carbide from tungsten hexafluoride, hydrogen and a carbon-containing gas has been described in, for example, international patent application WO 00/47796. The carbon-containing compound is initially thermally activated. All of the gaseous source chemicals are introduced into a reaction space at the same time, resulting in the deposition of nonvolatile tungsten carbide on the substrate. A CVD reaction of WF6 with trimethylamine and H2 has been shown to produce WC films at 700° C.-800° C. and beta-WCx films at 400° C.-600° C. (Nakajima et al., J. Electrochem. Soc. 144 (1997) 2096-2100). The H2 flow rate affects the deposition rate of tungsten carbide. A problem with the disclosed process is that the substrate temperature is rather high relative to thermal budgets for state-of-the-art semiconductor fabrication, particularly in metallization stages.
PVD processes generally deposit along a line-of-sight. One method of depositing tantalum carbide for a diffusion barrier layer by PVD has been described in U.S. Pat. No. 5,973,400. A tantalum carbide layer was formed by sputtering tantalum or tantalum carbide under an N2/CH4/Ar atmosphere. Line-of-sight deposition, however, means that complex substrate contours will have insufficient thin film coverage in shaded areas. Additionally, line-of-sight deposition means that low-volatility source material arriving directly from the source to the substrate will likely adhere to the first solid surface that it encounters, thus producing low-conformality coverage.
A “thermal” ALD method of forming metal carbide films, wherein the substrate is sequentially and alternately contacted with vapor phase pulses of two or more source chemicals, is described, for example, in U.S. Pat. No. 6,482,262. According to the methods described therein, a transition metal source chemical and carbon source gas are alternately and sequentially exposed to a substrate in a reaction space, which is maintained at an elevated temperature. The pulsing sequence is repeated to form a metal carbide (e.g., TaC) film of desired thickness. Due to the self-limiting nature of ALD, thin films are grown at about one monolayer (ML) increments. Thus, ALD has the potential for producing substantially uniform and highly conformal metal carbide films.
Problems with prior art methods of forming metal carbide films include difficulties in forming films with low impurity contents. Depending on the precursor used, metal carbon films may include high halogen, oxygen and/or carbon impurities, which significantly reduce film quality and device performance. Although ALD is capable of forming uniform metal carbide films on geometrically challenging structures, producing high quality (i.e. low impurity) films may be difficult to achieve with prior art methods.
Accordingly, there is a need for improved ALD methods of depositing metal carbide films with low impurity contents.